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If you're needing to speed-test your internet service then you can use web-based speed testers such as https://speedtest.aa.net.uk, but they may not always give you accurate results. An alternative is using a tool called iperf3 to run a speed test against an iperf3 server that is hosted at your ISP.

iperf3 is a program that is easily installed on many Linux distributions, but is a little harder to use on Windows.

Here is how to download and run iper3 on Windows and how to test single and multi-thread upload and download speeds. - This uses the Andrews & Arnold speed test server, which is only available to customers.

The version of iperf3 we have here is 3.13 from Github built with cygwin. It's a newer version that is hosted on the iperf.fr website and so has a lot of bug fixes. - Thanks to ''Thomas Jepp'' for compiling this.

===1) Download...===
Download '''[[Media:Iperf-3.13-windows-x64.zip]]''' and save it to your computer, eg in your Downloads folder. (The usual caveats apply when downloading files from untrusted sources...)

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Windows_iperf_1_download.JPG
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===2) Extract... ===
Right click on it, and select   '''Extract all'''.

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Windows_iperf_2_extract.JPG
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===3) Open Command Prompt...===
Open up the Command Prompt - Click  '''<SyntaxHighlight inline>Start</SyntaxHighlight>''' then type   '''<SyntaxHighlight inline>cmd</SyntaxHighlight>''' and click on the 'Command Prompt' icon.

===4) Find the files...===
In the black box type: '''<SyntaxHighlight inline>cd Downloads\Iperf-3.13-windows-x64\iperf-3.13-windows-x64  </SyntaxHighlight>'''

You can check if you're in the right place by typing '''<SyntaxHighlight inline>dir</SyntaxHighlight>''' and you should see some files including '''<SyntaxHighlight inline>iperf3.exe</SyntaxHighlight>''' listed.

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Windows_iperf_3_cmd_cd.JPG
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===5) Run iperf3 ===
Test it by running a simple speed test by typing:  '''<SyntaxHighlight inline>iperf3.exe -c speedtest.aa.net.uk</SyntaxHighlight>''' (you may need to click 'allow access' if a Windows Defender window pops up.

By default, this will run a single-thread upload speedtest (sending traffic TO the speed test server).  As you can see in the screenshot, the upload speed this line happens to get is just over 19Mb/s.

There are various other useful tests you can run, see the variations below.

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Windows_iperf_4_results.JPG
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===6) Send us the results===
You can copy all the text in the Command Prompt and email it over to us: Pressing '''<SyntaxHighlight inline>Ctrl-A</SyntaxHighlight>''' (The Control Key and the A key) will select all the text, and '''<SyntaxHighlight inline>Ctrl-C</SyntaxHighlight>''' will copy it to the clipboard. You can then open up your email and paste it in to the message.

=Variations=

There are variations you can do to test the speed as follows by using the '''-R''' (reverse) and '''-P''' Parallel options, we've also included a timestamp and a title to help with the logging:

* Run a '''single-thread  Download''' test:  <SyntaxHighlight inline>iperf3.exe --timestamps -T "1xDOWN" -R -P1 -c speedtest.aa.net.uk</SyntaxHighlight>
* Run a '''single-thread Upload''' test: <SyntaxHighlight inline>iperf3.exe --timestamps  -T "1xUP" -P1 -c speedtest.aa.net.uk</SyntaxHighlight>
* Run a '''10-thread Download''' test: <SyntaxHighlight inline>iperf3.exe --timestamps  -T "10xDOWN" -R -P10 -c speedtest.aa.net.uk</SyntaxHighlight>
* Run a '''10-thread Upload''' test: <SyntaxHighlight inline>iperf3.exe --timestamps  -T "10xUP" -P10 -c speedtest.aa.net.uk</SyntaxHighlight>

===What to look for:===
* The results should be near to the speed you'd expect for your line.
* You may want to run the tests at different times of day so as to see if there is any peak-time congestion affecting you.
* Hopefully the single and multi-thread tests give similar results - if not, there may be some sort of problem.